It's not legitimate because there's no plan for victory. We just see situations cropping up at random and deal with them. It generally doesn't work, and even if it did, terrorism is accepted. It's been around since ancient times, and despite our hatred of it, the US uses it regularly.

With WWII and the Civil War we knew who was the enemy and how to stop them. Even guerrilla wars can be stopped with special tactics, but we don't use them too well. That's why we're failing in Iraq. First, we never considered the implications of "freedom" for the Iraqis, and second we didn't know how to deal with the inevitability of post-Saddam resistance and lawlesness.

Any military commander with half a brain will tell you that in order to succeed, you have to stop the problem at it's source, not rely on the enemy's first strike to dictate your actions.